The present invention relates to actively damping the effects of rack disturbances on an EPS system.
Electric Power Steering (EPS) systems use an electric motor to provide assist to the driver and to mitigate road disturbances. Linear system and control analysis techniques can be used to recommend calibrations for optimal steering feel and road disturbance rejection.
Due to physics of an electric motor, available motor torque can decrease as the motor velocity increases. This is typically described by a motor envelope curve that represents available motor torque versus motor velocity plot as shown in FIG. 1A. Also, most of the driving/operating conditions for an EPS system typically occur within a region of a motor envelope closer to origin, i.e. maximum available motor torque is more or less constant as shown in FIG. 1A. However, an EPS system can be subjected to higher than nominal rack loads, causing the system to operate at much higher velocities. For example, in one experiment, driving on an off-road surface with sudden braking lead to 50% higher than typical steering loads which lead to 100% higher than typical motor velocities. This is a complex nonlinear phenomenon that involves factors including an EPS mechanical design, motor design, motor control & calibrations, EPS control & calibrations, chassis dynamics etc. These high rack loads can cause high motor velocity in an EPS system, reducing maximum available motor torque in the process as shown in FIG. 1A. If the combination of high motor velocity and reduced maximum available motor torque occurs as shown, for example, in FIG. 1A, the motor may not be able to produce sufficient reaction torque to match the rack load acting on EPS system. This may lead to an additional increase in motor velocity. This cycle continues, leading to undesirable, high motor velocities in an EPS system. A challenge is to recognize this phenomenon and apply motor reaction torque quickly (e.g.) before motor velocity further increases. There are different ways to mitigate this phenomenon.
One way to mitigate this phenomenon is to modify an EPS mechanical design to reduce the high motor velocities in the system. However, this technique can cause degradation in steering feel. A larger capacity motor can be used to provide more reaction torque but this would mean significantly more cost. Limitations of both of these methods make an active damping algorithm desirable option for mitigating effects of high rack loads.
Active damping is typically used in an EPS system to improve steering performance. U.S. Pat. No. 5,919,241 describes the use of active damping based on steering velocity and other signals to achieve desired frequency response and performance of an EPS system. U.S. Pat. No. 6,647,329 B2 and U.S. Pat. No. 6,122,579 also describe different strategies for a damping function. U.S. Pat. No. 8,612,094 B2 presents a strategy to scale frequency dependent motor damping based on the velocity signal itself. U.S. Pat. No. 7,549,504 B2 and US 2009/0157258 A1 present a methodology for applying active damping, to an EPS system, that can operate differently in different quadrants, where a quadrant is defined based on steering wheel torque and angular velocity.
Generally speaking, active damping strategies are often used is EPS systems to give optimal steering feel and road disturbance rejection for typical driving conditions. In these conditions, rack loads are within the typical operating range of an EPS system and the motor velocities are lower (typical driving region in the Motor Envelope FIG. 1A). In order to prevent high motor velocities (as described above) a much higher amount of damping is desired. Typically, such damping magnitudes will not be desired for normal driving conditions. Hence, these high damping magnitudes should be used outside the typical driving operating range and upon detection of the phenomenon described above. The detection of this phenomenon is crucial in order to mitigate such rack disturbances.